I'm thinking that when Americans don't want to talk about race and racism, it's because the subject is actually making them uncomfortable because it's such a large part of the American experience, and connected to harmful irrational feelings that even well-intentioned Americans absorb from the culture, and have to fight. Whereas when they don't want to talk about class, it's because they honestly think they live in something like a classless society.

Which from, say, a British perspective, may even be partly true: here class is mostly about money, whereas over there it has another dimension that strikes me as similar to our fixation on race. Which is not to say that it's completely absent here; we have the distinction between "new money" and "old money"; but it's nowhere near as powerful in people's minds, and race occupies that brain lobe instead.